Estimating groundwater response time in humid climate by using spectral analysis

During dry periods, river flow comprises baseflow, which typically generates from shallow aquifers. Understanding how such aquifers respond to climate events is key to managing environmental issues related to water supply or water quality. A typical indicator of groundwater response to climate events is the characteristic response time, which indicates the rate of depletion of shallow aquifers.

The traditional method to infer the characteristic response time analyzes the slope of the hydrograph recession curve. Such a method does not account for stormwater contribution in recession analysis, thereby assuming that the catchment is dry and the only contribution to discharge originates from groundwater. As a consequence, the recession analysis might underestimate the groundwater response time, owing to the presence of faster discharge components, i.e. surface runoff or interflow, in the falling limbs.

In this work, we propose an alternative methodology to calculate the characteristic response time, which is determined by analyzing the behavior of the baseflow time series in the frequency domain. The aquifer can be conceptualized as a low-pass filter, which smooths the high-fluctuating components in the recharge signal. Such behavior causes a cut-off frequency in the baseflow spectrum, which corresponds to the aquifer characteristic time. We applied this approach to several gauging stations in Germany, whose humid climate is ideal to compare the results with the classical recession analysis.

We observed that spectral analysis yields characteristic response times systematically larger than the ones calculated with recession analysis. On average there is a factor of two between the estimates provided by the two methods. Overall our study emphasizes careful consideration of the estimation of groundwater response times, especially in humid and sub-humid river basins.